Process for manufacturing high-strength,wear-resistant piston rings



United States Patent 3,518,128 PROCESS FOR MANUFACTURING HIGH- STRENGTH, WEAR-RESISTANT PISTON RINGS Tsutomu Takao, Kawaguchi-shi, and Kentaro Takahashi, Ohmiya-shi, Japan, assignors to Nippon Piston Ring (10., Ltd., Tokyo, Japan No Drawing. Filed June 29, 1967, Ser. No. 649,865 Claims priority, application Japan, July 23, 1966, 41/ 48,226 Int. Cl. C21d 5/14 US. Cl. 148-3 4 Claims ABSTRACT OF THE DISCLOSURE A process for manufacturing high-strength, wearresistant piston rings by pouring a green ferrous material in a molten state consisting of 2.0-3.2% carbon, 0.6- 1.5% silicon, 1% or less manganese, 1.0% or less phosphorus, 0.3% or less sulphur, 0.002-0.04% boron and (Ll-0.3% chromium, and the balance iron, into molds and leaving it as cast therein so that it may structurally be formed into white cast iron, removing the molded ferrous material from the molds and then subjecting the thus-obtained castings to specific heat treatments, such as tempering, reheating, quenching and the like, to thereby obtain high-strength, wear-resistant piston rings.

BACKGROUND OF THE INVENTION As is generally known, conventional piston rings used for internal combustion engines are gradually worn by sliding movement within a cylinder liner simultaneously with wearing of the mating liner. However, it is necessary that the piston rings themselves should be substantially wear-resistant, while they should not greatly wear the cylinder liner in which they move slidably by friction with the liner. Materials for piston rings, which meet the above requirements, normally include unalloyed cast iron, alloy cast iron, ductile cast iron and malleable cast iron, and further include these metals which are surfaced, for example, plated with chromium.

Malleable cast iron has good castability and machinability, and also has high strength compared with unalloyed cast iron. Malleable cast iron is thus usable as an excellent material for piston rings if it is processed to be more wear-resistant. Factors upon which the wearresistance required by piston rings depends, are the shape, size and quantity of graphite and carbide in the material composing the piston rings, and the matrix of the material.

It is necessary that a ferrous material for piston rings should contain free graphite of a suitable size and shape, and carbide in a suitable amount; and it is desired that the matrix of the material be pearlitic or sorbitic.

The ferrous material for piston rings of this invention contains boron and chromium in specific amounts, respectively.

Chromium, when added to malleable cast iron, is intended to prevent ferrite from precipitating so that the malleable cast iron is made pearlitic. The amount of chromium added should not exceed a certain level, because chromium will combine with carbon in an iron melt containing the carbon to form carbide and con- 3,518,128 Patented June 30, 1970 sequentially harden the iron when added to the iron bath and because it will preclude graphite formation when added to malleable cast iron.

Boron is an element which has as high a carbideforming capability as chromium, and will have a delicate effect on the time for graphitizing malleable cast iron when added thereto; it will accelerate the graphitization when added to malleable cast iron in an extremely small amount, while it will preclude graphitization when added in a larger amount.

It has now been found that there can be produced machine parts such as piston rings, which have higher wear-resistance and strength, by carrying out the process of this invention.

An object of this invention is to provide an improved malleable cast iron for machine parts such as piston rings, which is superior in strength and wear-resistance.

Another object of this invention is to provide a process for the production of high-strength, wear-resistant piston rings from said improved malleable cast iron.

Still another object of this invention is to provide highstrength, wear-resistant piston rings.

The reproduced photograph shows, in magnified form, the structure of a piston ring material according to this invention. As is seen from the photograph, the material is characterized by the presence of fine granular graphite and carbide in a suitable amount. The presence of the fine granular graphite increases the strength of the material and that of the carbide in a suitable amount improves the wear resistance of the material. The carbide content in a cast iron of this invention can easily be controlled by incorporating chromium and boron with the cast iron.

The piston rings of this invention are produced ac cording to the following procedures.

There is firstly prepared a green ferrous material in a. molten state comprising 2.0-3.2% carbon, 0.6-1.5 silicon, up to 1.0% manganese, up to 1.0% phosphorus, up to 3% sulphur, 0.002-0.*04% boron and (Ll-0.3% chromium, and balance iron.

The molten green ferrous material is then poured into a mold for piston rings and left as it is cast in the mold so that it forms white cast iron in structure.

The term a green ferrous material used herein means a ferrous material which has not yet been subjected to the heat treatments according to this invention.

(1) The piston ring castings released from the mold are maintained at 900-1000 C. for from 20 minutes to 3 hours in a furnace to graphitize about -85% of the combined carbon initially present in the castings, allowed to cool to about 800 C. and maintained at this temperature for 5-30 minutes, and then cooled in the furnace or in air outside the furnace so that they may have a pearlitic matrix. Alternatively, subsequent to the graphitization described above, the castings are allowed to cool to room temperature, reheated to about 800-850" (3., maintained at this temperature for 5-30 minutes and then cooled in the furnace or in air outside the furnace so that their matrix may be made pearlitic.

(2) Or the piston ring castings as released from the mold are at first maintained at 900-1000 C. for from 20 minutes to 3 hours and then at 800-850 C. for 5-30 minutes, and thereafter quenched in oil or water so that they may have a martensitic matrix, and then tempered at IOU-600 C. for 0.5-3 hours. Alternatively, after said graphitization the castings are allowed to cool to an ambient temperature, reheated to SOD-850 C., maintained at this temperature for -30 minutes, quenched in oil or water so that their matrix may be made martensitic and then tempered at 100600 C. for 0.5-3 hours. The castings thus heat-treated are then machined to produce the finished piston rings.

According to this invention, the amounts of boron and chromium contained in the ferrous material for piston rings should be limited to 0.002-0.04% and 0.1-0.3% of the ferrous material, respectively. The reasons for the limitation will hereinafter be discussed.

Boron will have very delicate effects on the structure of the ferrous material even if contained in an extremely small amount ,in the material. More particularly, boron will promote graphitization simultaneously with precipitating and uniformly distributing carbide in the ferrous material when present therein in a small amount, while it will preclude graphitization and at the same time precipitate too much carbide if it is contained in the material in too large an amount. Boron should therefore be contained in the ferrous material in amounts of 0.04% to 0.02% of the material, the 0.002% being the smallest amount for the boron to have a practical effect on the structure of the material.

Chromium is inherently an element which makes ferrite vanish and stabilizes pearlite when it is present in a ferrous material. Chromium will stabilize carbide too securely whereby the rate of graphitization is retarded and consequently too much carbide will be left unchanged if it is contained in too large an amount in the ferrous material, while it will have no advantageous effects on the properties of the material if contained in too small an amount in the material. It has thus been found that the amount of chromium to be contained in a green ferrous material according to this invention should be between 0.3% and 0.1%, based on the weight of the material.

It has also been discovered that the presence of 0.002- 0.04% of boron and 0.1-0.3% of chromium in the green ferrous material according to this invention results in 60- 85% of the original combined carbon being graphitized, when the material is tempered at 900-1000 C. for /s3 hours. The material, the structure of which has been modifled by graphitization is superior in strength and wear resistance. It is only when the green material contains both chromium and boron in the respective amounts mentioned above that the structure of the material can be changed to the desired one as described above by graphitizing the material during such tempering treatment.

On the other hand, if the green material contains only industrially be more diflicult to change to the desired structure because of a difficulty in selecting suitable tempering conditions corresponding to the content of the element in the material. If the green material should contain only chromium and be tempered, even a slight increase in chromium content will result in leaving too much combined carbon unchanged in the material, While a low content of chromium will make it difiicult to allow combined carbon to remain in a necessary amount in the material. Thus, the combined use of chromium and boron in ferrous materials has made it possible to produce very excellent ferrous materials for piston rings in an industrially easy and economical way.

The tempering temperature for the green ferrous material according to this invention should be up to 1000 C. because temperatures above 1000 C. rough the graphite structure of the tempered material and deteriorate the wear resistance thereof. The green material will take more than 3 hours to be tempered when tempered at less than 900 C., and this is operationally and economically dis advantageous. The material should be tempered for %3 hours because it will decrease to less than 40 %in amount of residual combined carbon when tempered for less than minutes, whereas 85% of the combined carbon in the green material will be graphitized when the material is 4 tempered for more than 3 hours. The green material of this invention should thus be tempered so that 60-85% of the combined carbon present therein is graphitized, because it will deteriorate in machinability and be inferior in workability if less than 60% of the carbon be graphitized thereby to produce graphite in an amount insufficient for the purpose of this invention. On the other hand, the material Will decrease in wear resistance if the combined carbon be left unchanged in an insufficient amount in the tempered material by graphitizing more than of the total combined carbon present in the green material.

The material tempered is then required to be subjected to a cooling treatment in a furnace or in air outside the furnace to make it pearlitic. In this treatment it should be cooled at a cooling rate in a certain range depending on its composition in order to make a pearlite of it. It will be martensitic and hardened if cooled too quickly, while it will be ferritic and softened if cooled too slowly. It should therefore be cooled at a reasonable cooling rate according to its thickness in a furnace or in air outside the furnace so that it maybe pearlitic. The material will, however, be martensitic owing to rapid quenching when quenched in oil or water. The martensite thus produced will then be made a tempered sorbite when further tempered. The thus-obtained sorbite is superior in toughness and Wear resistance.

The material thusly heat-treated should finally be again tempered at more than C. because it will not entirely be freed of the strains caused by quenching and will be brittle if tempered at a temperature of less than 100 C. The material will be excessively softened and consequentially decreased in wear resistance if tempered at a temperature of more than 600 C.

The presence of both chromium and boron in the green ferrous material according to this invention, when the material is heat treated as previously mentioned, will make it easy to render the material pearlitic or sorbitic and will make it possible to precipitate fine graphite particles of a suitable size and carbide in a suitable amount without leaving free ferrite in the material.

Piston rings which are excellent in strength and wear resistance can be obtained from the green ferrous material by subjecting it to such heat treatments as men tioned above.

This invention will be better understood by the following examples.

Example 1 In an arc-type electric furnace there is melted a green ferrous material having the following compositions:

T.C. 2.38 Si 1.42 Mn 0.37 P 0.13 S 0.041 B 0.01 Cr 0.2 Fe Balance The molten, green ferrous material is heated to 1520 C. and cast in green sand molds for piston rings of Some of the castings as removed from the molds in Example 1 are maintained at 980 C. for one hour, cooled to 830 C. and maintained at this temperature for 15 TABLE 1 Castings Castings heat-treated heat-treated Properties in Example 1 in Example 2 Amount of graphite (percent) 68 75 E modulus (kg/mmfi)... 16,200 16, 300 Tensile strength (kg/mm 62 70 Hardness (HnC)- 32 43 What is claimed is:

1. A process for manufacturing high strength, wearresistant piston rings comprising pouring into a mold for piston rings a green ferrous material in the molten state comprising 20-32% carbon, 0.6-1.5% silicon; 1.0% or less manganese, 1.0% or less of phosphorus, 0.3% or less of sulphur, 0.002-0.04% boron and 01-03% chromium, and the balance iron, leaving same as cast'so that it may be structurally made into white cast iron, removing the resulting piston ring castings from the molds, maintaining the same at 900-1000 C. for /2'-3 hours in a furnace to graphitize about 60-85% of the combined carbon of the castings, cooling same to about 800 C. and maintaining it at said temperature for 530 minutes and then further cooling at a rate sufficient to produce high strength, wear-resistant piston rings having a pearlitic matrix.

2. A process for manufacturing high strength, wearresistant piston rings comprising pouring into a mold for piston rings a green ferrous material in the molten state comprising 2.0-3.2% carbon, 0.6-1.5 silicon, 1.0% or less of manganese, 1.0% or less of phosphorus, 0.3% or less of sulphur, 0.002-0.04% boron and 0.1-0.3% chromium, and the balance iron, leaving same as cast so that it may be structurally made into white cast iron, removing the resulting piston ring castings from the molds, maintaining same at 900-4000 C. for /a-3 hours in a furnace to graphitize about 60-85% of the combined carbon of the castings, cooling same to an ambient temperature, reheating it to about 800*850 C. and maintaining same at said temperature for 5-30 minutes and then cooling at a rate sufficient to produce high strength, wear-resistant piston rings having a pearlitic matrix.

3. A process for manufacturing high strength, wearresistant piston rings comprising pouring into a mold for piston rings a green ferrous material in the molten state comprising 2.0-3.2% carbon, 0.6-l.5% silicon, 1.0% or less of manganese, 1.0% or less of phosphorus, 0.3% or less of sulphur, 0.002-0.04% boron and 0.1- 0.3% chromium, and the balance iron, leaving same as cast so that it may be structurally made into white cast iron, removing the jiresulting piston ring castings from the molds, maintaining' same at 900-1000 C. for /a-3 hours in a furnace to graphitize about -85% of the combined carbon of the castings, cooling same to 800- 850 C. and maintaining same at said temperature for 5-30 minutes, quenching the castings in oil or water at a rate suflicient to render the castings martensitic in structure and then tempering the quenched castings at 600 C. for Vz-3, hours to produce high strength, wear resistant piston rings having a sorbitic matrix.

4. A process for manufacturing high strength, wearresistant piston rings comprising pouring into a mold for piston rings a green ferrous material in a molten state comprising 20-32% carbon, 0.6-1.5% silicon, 1.0% or less of manganese, 1.0% or less of phosphorus, 0.3% or less of sulphur, 0.0020.04% boron and 0.1-0.3% chromium, and the balance iron, leaving same as cast so that it may be structurally made into white cast iron, removing the resulting piston ring castings from the molds, maintaining same at 900-1000" C. for /3-3 hours in a furnace to graphitize about 60-85% of the combined carbon of the castings, cooling same to an ambient temperature, reheatingit to 800-850 C. and maintaining same at said temperature for 5-30 minutes, quenching the castings in oil or water to render the castings martensitic in structure and then tempering the quenched castings at IOU-600 C. for /&--3 hours to produce high strength, wear-resistant piston rings having a sorbitic matrix.

References Cited UNITED STATES PATENTS 1,871,543 8/1932 McCarroll et al. 148-3 2,579,452 12/1951 Eckman et a1. 148138 X 3,419,439 12/1968 Burgess 148-139 X CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 

